Nuclear reactors conventionally comprise a pressure vessel containing a core formed from a large number of fuel assemblies, each assembly comprising top and bottom end pieces connected together by tubes, of which some may be used as control rod guide tubes, with a plurality of interspaced spacer grids mounted by the tubes between the end pieces. These spacer grids have a large number of openings through which metal clad fuel rods are inserted with the grids holding the fuel rods properly interspaced laterally and parallel to each other.
The spacer grid openings have larger cross sectional sizes than the fuel rods, and contact elements are used between the fuel rods and the spacer grid surfaces defining the openings in which the fuel rods are inserted. In the case of a pressurized-water reactor, the operating temperatures involved permit these contact elements to be designed as resilient elements and during thermal movements of the fuel rods relative the spacer grids, the intercontacting surfaces formed by the fuel rods and the elements remain relatively slidable so that the fuel rod metal claddings are not appreciably stressed, because the fuel rods can move relative to the spacer grids and the contact elements. However, in the case of gas-cooled nuclear reactors, core operating temperature are considerably higher than are involved by the operation of a pressurized-water reactor, the fuel rods and spacer grids being subjected to temperatures commonly ranging between 300.degree. and 600.degree. C. Resilient contact members are, of course, made of elastic or spring metal, and therefore, cannot remain capable of providing resilient fuel rod centering forces, at such higher temperature. Therefore, solid and rigid metal contact elements must be used to center the fuel rods in the openings formed by the spacer grids.
It has been found that with the fuel rod metal claddings and such solid and rigid metal contact elements forming rigidly fixed rubbing surfaces, the surfaces, instead of relatively sliding, tend to stick together. With the fuel rods unable to move relative to the spacer grids, the fuel rod claddings become highly stressed. This is undesirable because if a fuel rod cladding ruptures, the nuclear fuel is released.
It can be seen that in the case of gas-cooled reactors, a problem exists in connection with centering the fuel rods within the spacer grid fuel rod openings, without the fuel rod claddings becoming undesirably highly stressed during operation of the reactor.